Biosolids are residues generated through the treatment of sewage. Biosolids are characterized by containing inorganic and organic compounds precipitated during wastewater treatment. Beneficial uses of biosolids for the nutrient content and soil amendment properties have been recognized for agricultural and recreational gardening. In spite of initial concerns as to pathogen and contaminant content of biosolids, it has been found over the past forty years of research that agronomic use of high-quality biosolids is sustainable and safe.
Application of biosolids to land either to condition the soil or fertilize vegetation affords a double benefit in enhancing soil quality while providing a less expensive alternative method of disposal for the biosolids. While biosolids typically contain appreciable amounts of essential nutrients for plant growth, and especially nitrogen and phosphorus, biosolids are characterized by generally being low in potassium as the potassium most often remains in the effluent discharge of sewage treatment. In addition to nitrogen and phosphorus, biosolids contain other essential plant nutrients such as calcium, magnesium, sulfur, boron, copper, iron, manganese, molybdenum, and zinc.
Agricultural use of biosolids while reducing fertilizer cost includes a degree of variability associated with weather conditions and crop type. Transport of biosolids and application at times that are compatible with agricultural schedules of planting, harvesting, and possible adverse climactic conditions requires careful management; and when conditions are not favorable, biosolids have traditionally been disposed of through more expensive and environmentally deleterious options such as landfill or incineration.
In response to these problems associated with biosolids, it has become more common to dry and pelletize biosolids. The city of Milwaukee has sold Milorganite® since 1926 and Granulite® that has been sold by Synagro Technologies since 1992. Dry biosolids are amenable to blending with other materials when used as organic-based fertilizer with balanced nutrient levels. Alkaline stabilization has been used to produce a nearly odorless granular product for use as a soil conditioner, fertilizer, and also as a substitute for agricultural lime. Unfortunately, alkaline stabilization or other processes of blending has proven complex and expensive while not reducing the propensity of dry biosolids to spontaneously undergo exothermic biological and chemical heating. While prior art attempts to enhance the fertilizer properties of biosolids through addition of a liquid spray and/or dry formulations applied to the biosolids, these have met with limit success owing to excessive wetting converting the biosolids to a nonspreadable sludge with limited amounts of soluble material being applied. Additionally, liquid application of supplemental potassium in particular is limited in the quantity of potassium that can be so provided. Solid additives are prone to forming inhomogeneous compositions through dusting and the density differential between the biosolids and solid additives.
Thus, there exists a need for enhanced biosolids pellets having high surface loadings of potassium salts and a process of forming biosolids pellets that have been nutrient enriched to form a balanced fertilizer while maintaining pellet stability.